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Modern motherboards can be any size or shape that suits the design of the PC. Only the need for standard size expansion slot connectors limits the freedom of design. Many PCs, however, are built around motherboards of a few standard sizes. This standardization is a matter of convenience. It allows a PC manufacturer flexibility in the choice of suppliers-standardized dimensions make motherboards interchangeable.

For you as the purchaser of a new computer, motherboard standardization has its downside. You face the problem of the PC manufacturer selling systems equipped with a motherboard du jour, whatever OEM motherboard was available cheapest on the day the PC was put together. On the other hand, a PC built around a standard-sized motherboard gives you upgrade freedom. Should you become dissatisfied in the performance of your PC, you can replace a standard size motherboard with a more powerful one.

The earliest motherboard standards followed the leads set by IBM. They duplicated the physical dimensions of the motherboards used by the most popular IBM machines. Even when they lopped off vast areas of board to trim costs, most manufacturers retained compatibility with IBM's designs, keeping mounting holes in the same locations so that one board could be substituted for another. This heritage continues even in some of the latest designs.

After IBM ceded its influence as the setter of the standard dimensions of motherboards, the industry was essentially adrift. Major manufacturers developed their own designs without regard to older products, while smaller manufacturers clung to the old board layouts. The situation is changing, though, with new motherboard standards now promulgated by Intel. The most recent of these, the ATX motherboard design, goes further than ever before and specifies not only dimensions and mounting holes but also connector placement and even connector designs.

Motherboard design divergence first arose among makers of small-footprint PCs, machines designed with smaller dimensions to cover less of your desktop. These machines compromised expansion by reducing the number of expansion slots and drive bays to gain their more modest measurements. Many manufacturers developed low profile PCs that reduced system height by turning expansion boards on their sides. These designs necessitated changes from the more standardized motherboard layout. Another new standard, the LPX motherboard, now provides a uniform platform for low profile computers.

In general, smaller manufacturers are more likely to use standard size motherboards. Larger manufacturers are better able to afford the price of custom designing cases and motherboards to match. Even large PC makers have moved to standard size boards, at least for their offerings that take advantage of the latest microprocessors. In truth, systems that use the latest microprocessors often all have exactly the same motherboard design inside, using a motherboard designed and manufactured by Intel. Even large manufacturers may rely on Intel motherboards until their engineers develop a familiarity with new chips. For example, all but a handful of the first Pentium Pro computer models uniformly used standard size Intel-manufactured motherboards.

Early Motherboards

The first motherboard to be used in PCs was that of IBM's original Personal Computer. These measured about 8-1/2 by 11 inches and had five expansion slots spaced one inch apart in the left rear corner of the board.

This original PC design is essentially irrelevant to today's PC market. No other manufacturer matched its exact design because its layout allowed only five expansion slots. When nearly every function had to be located on its own expansion board, as was the case in the first few years PCs were sold, five slots quickly proved inadequate. Nevertheless, even though the exact layout proved short-lived, the PC motherboard design set the pattern for the vast majority of motherboards for PCs for more than a decade. Nearly all new motherboards put their power connectors and keyboard connectors in the same places. The space occupied by the expansion slots is typically the same, too, although the type of connectors used has changed (the original PC had only an eight-bit expansion bus) as well as their spacing.

The first lasting standard came out of IBM's second iteration of PC design back in 1982 with the development of the XT motherboard. To accommodate a bit more circuitry, IBM extended the board to 8.5 by 12 inches. More importantly, IBM squeezed the expansion slots closer together, to increments of 0.8 inch, to allow eight slots to fit in the place of five. This slot spacing has since become the standard among nearly all PCs. It defines the maximum width permitted for expansion boards.

Some modern motherboards using the most advanced microprocessors still use the basic XT dimensions. Although these motherboards now have different arrangements of expansions slots to allow for the use of high speed expansion buses like PCI, they still maintain the 0.8-inch slot spacing.

The IBM design put screws only in two of the mounting holes of the XT motherboard. The other holes use special insulation stand-offs that allowed the board to slide into place.

AT

The most popular motherboard layout originating from IBM was that of the Personal Computer AT, introduced in 1984. This layout is still used by many manufacturers and has become accepted as an industry standard.

Manufacturers prefer the AT layout when they require more space for more circuitry because it is the largest design that's widely used. The original AT motherboard measured 12 by 13 1/2 inches, and this size is still used for motherboard designs that require the largest possible area for the installation of circuit components. Often the first generation of PCs using a given microprocessor uses this size board because new chips usually require many discrete chips for their support circuitry.

As with the XT layout, the AT motherboard design allows for eight expansion slots spaced at 0.8-inch increments. Other than the placement of the expansion slots, mounting holes, and (usually) the keyboard jack, manufacturers follow no standard in their component layout. You may find microprocessor and memory at any place on the board. In poorer designs, for example, the memory sockets may interfere with the installation of full-length expansion boards.

Most AT motherboards provide I/O ports, if at all, in the form of headers, special inexpensive connectors soldered to the motherboard. System makers link connectors on the chassis to the motherboard headers with short ribbon or twisted-wire cables. Some AT motherboard makers permanently attach port connectors to the rear of the board, although they may put the connectors in any arrangement that suits their fancy-and the particular case they choose to use.

Mini-AT

The primary way PC manufacturers reduce the cost of their products is by taking advantage of increased circuit integration. Once a given class of microprocessor has been on the market for a while, typically a year or two, chip-makers develop the technology to reduce the number of chips required on a motherboard. Instead of several dozen, the principal circuitry of the motherboard may be reduced to three or four chips. Fewer chips require less circuit board space. By making the motherboard physically smaller, manufacturers can reduce the cost of the materials to produce the motherboard. This gives manufacturers incentive to make miniaturized motherboards.

Because the placement of mounting holes and the spacing of expansion slots of the AT motherboard have become essentially industry standards over the years, engineers have crafted smaller motherboard designs that retain these features. These smaller AT-compatible designs are termed Mini-AT motherboards.

Although the exact dimensions vary with the board maker, a typical Mini-AT motherboard measures about 13 inches by 8.7 inches (330 by 220 millimeters) with the expansion slots running parallel to the long axis of the board.

As with full-size AT motherboards, mini-AT boards are typically designed to fit a variety of cabinets and connector arrangements. Consequently, most mount only the keyboard connector directly on the printed circuit board. Other port functions terminate in headers, and short ribbon cables connect these headers to the external connectors mounted on the PC chassis.

LPX

One thing stands in the way of making a low profile PC-the height of the expansion boards. You can't make a system shorter than the height of an expansion board, at least if you have hopes of installing standard expansion boards into it.

Shorter PCs have their allure. Compact systems are less intimidating, more stylish, and even make good monitor stands. To push down the top of the case of their systems, most manufacturers resorted to turning the expansion boards (and, of course, their mating connectors) on their sides inside the system chassis. The result was a profusion of odd-shaped motherboards with a number of different arrangements of their expansion provisions.

Bringing sense to this chaos and helping you find a replacement motherboard that fits most systems, the PC industry has developed a standard form factor for low profile motherboards. Called LPX, the standard low profile board measures 8.66 by 13 inches (or 220 by 330 millimeters), essentially the same dimensions as a mini-AT board. The short dimension of the board runs parallel to the rear of the PC chassis and hosts the connectors for input and output ports. In fact, the chief difference between an LPX motherboard and a mini-AT design is the form and placement of the expansion connectors.

In place of expansion slots, the LPX motherboard has a single master expansion connector near the rear of the board, just to the right of its centerline. A small daughtercard plugs into this slot and holds one or more standard expansion connectors, which follow the ISA or PCI standard (or both). Guides in the chassis secure the boards in the slots so that they are parallel to the motherboard.

The keyboard connector on an LPX motherboard keeps to the same location as that of the mini-AT design. I/O connector placement is not part of the LPX layout, so different manufacturers locate port connectors wherever they see fit.

Mini-LPX

To save both cabinet space and the cost of materials, some manufacturers trim the LPX-size motherboard to even smaller dimensions to create a Mini-LPX board. In all other ways much the same as a full-size LPX board, the miniaturized design lops about three inches from the long axis of the board. The result measures about 10 inches by 8.66 inches.

One example of the mini-AT motherboard is Intel's Advanced/MN design that is found in some low profile Pentium PCs.

ATX

To bring a degree of uniformity to motherboard design, the PC industry created a new motherboard standard that roughly conforms to the mini-AT board size but with a few design twists that result in lower cost engineering. Called ATX, the standard is promulgated by Intel but is openly published. Intel released the most recent version, 1.1, in February, 1996, fine-tuning the design based on industry feedback.

The principal design twist of the ATX board is that it gives a 90-degree turn to the Mini-AT design, putting the long axis of the board parallel to the rear panel of the host PC chassis. The ATX standard defines the number and position of the motherboard mounting holes and offers recommendations as to component, expansion board, and port connector placement. Although the standard does not demand any particular slot type or configuration, it's aimed primarily at ISA, PCI, and ISA/PCI combination designs. It also allows for both 5.0 and 3.3 volt system operation (or both simultaneously).

The odd orientation of the board facilitates port placement. It provides the maximum space for expansion boards and port connectors at the rear of the host PC chassis. The design also envisions that the microprocessor will be located near the right edge of the board where it will be in close proximity to both the power supply and cooling fan. In the recommended configuration, memory sockets can be readily accessed between the microprocessor and expansion slots.

The ATX board itself measures a maximum of 12 by 9.6 inches (305 by 244 millimeters). This size is not a random choice but, according to Intel, was selected to allow manufacturers to cut two boards from a standard size 18 by 24-inch raw printed circuit panel. It provides sufficient space for about seven expansion slots, which are spaced at the conventional 0.8 inches apart. It incorporates nine mandatory and one optional mounting hole, most of which are in the same positions as the holes in a Mini-AT motherboard.

The ATX specification goes further than simply indicating mechanical board dimensions. The standard also embraces the PS/2 size of power supply and specifies a new motherboard power connector.

Besides uniformity, the ATX design aims at trimming costs for PC makers. By putting port connectors on the motherboard, even in multiple layers, it eliminates the cost of connecting cables as well as the labor required for assembly. Eliminating cables also helps minimize potential radio frequency interference. In its recommended configuration, the ATX layout also allows the use of shorter floppy and hard disk connecting cables, with similar benefits. The power supply choice and location also trims cost for the PC manufacturer and helps the PC run cooler and even quieter.

Mini-ATX

The designers of the ATX board realized that the one certainty in PC circuit design is that functions get combined and made more compact. Just as all the circuitry on the AT-size board was shrunk to fit the mini-AT board, the ATX designers imagined that soon much of the ATX real estate would be superfluous. In that one of the primary goals in the design of ATX was trimming costs, it figured that trimming motherboards to a size smaller than ATX, as the technology permitted it, would reap savings in material costs. Consequently, they included a standard size for Mini-ATX motherboards in the ATX specification.

The Mini-ATX design chops the ATX motherboard down to 11.2 by 8.2 inches (284 by 208 millimeters). When installed in a PC, the Mini-ATX motherboard still sits at the rear edge of the chassis so that port connectors can be mounted directly to it without cables. In most chassis, the left edge still aligns with the left side of the case to allow space for a full complement of expansion boards. Because of this placement, the smaller size of the Mini-ATX board cuts off one row of ATX mounting holes. As a result, the lower row of mounting holes are displaced on the Mini-ATX design.

The Mini-ATX design has one chief benefit. It reduces the material costs for a motherboard by about 30% when compared to a full-size ATX board.